Field of the Invention
The present invention relates to a sheet processing apparatus that performs predetermined post-processing on sheets, and an image forming system including the sheet processing apparatus.
Description of the Related Art
Conventionally, there has been proposed a sheet discharge unit that detects a height of sheets stacked on a stacking tray, and determines whether or not the height of the sheets reaches a predetermined height to thereby detect stack overflow (see Japanese Patent Laid-Open Publication No. H02-270762). Further, there has been proposed an image forming apparatus that counts the number of sheets discharged onto a stacking tray, and compares the counted number with a limit number of discharged sheets to thereby detect overflow of sheets on the stacking tray (see Japanese Patent Laid-Open Publication No. H03-013454).
However, sheet processing apparatuses configured to discharge sheets onto a stacking tray include one provided with a stacking tray designed to have a sheet stacking surface bent in a sheet discharging direction.
FIGS. 8A and 8B are cross-sectional views of the stacking tray provided for the sheet processing apparatus, which has the sheet stacking surface bent in the sheet discharging direction. In this sheet processing apparatus provided with the stacking tray designed to have the sheet stacking surface bent in the sheet discharging direction, when a sheet having high rigidity is discharged onto the stacking tray, the sheet may not be stacked along the sheet stacking surface.
FIG. 8A shows a normal sheet stacking state in which sheets are stacked along the sheet stacking surface. On the other hand, FIG. 8B shows an abnormal sheet stacking state in which sheets are not stacked along the sheet stacking surface.
In general, on the sheet stacking surface of the stacking tray, there is provided a sheet presence sensor 715 that detects a sheet. However, in the abnormal stacking state in which sheets are not stacked along the sheet stacking surface, a sheet is not brought into contact with the sheet presence sensor 715, and hence the sheet is sometimes not detected with accuracy. In this case, it is impossible to determine whether sheets on the stacking tray have been removed by a user, or the abnormal stacking state in which a sheet cannot be normally detected has occurred, and hence it is difficult to detect stack overflow based on the number of stacked sheets and the like.
Further, since the sheets are not stacked along the sheet stacking surface of the stacking tray, a sheet stacking failure is highly likely to occur. When a sheet stacking failure occurs, sheets tend to fall, and if sheets continuously fall, it is difficult to detect stack overflow based on the height of stacked sheets.